The present invention generally relates to magnetic sensors and more particularly to a giant magneto-resistive device and a magnetic head using such a giant magneto-resistive sensor.
Magneto-resistive sensors are used extensively in magnetic heads of various conventional magnetic disk drives for reading information from a magnetic track formed on a magnetic disk.
A giant magneto-resistive sensor is a magnetic sensor having a superior magnetic sensitivity over an ordinary magneto-resistive sensor and is used in high-density magnetic disk drives. A typical example of a giant magneto-resistive sensor is a spin-valve sensor that can provide a magneto-resistive ratio exceeding 6%. A spin-valve sensor detects a magneto-resistance between a ferromagnetic free layer having a variable magnetization and a ferromagnetic pinned layer having a pinned magnetization, wherein the pinning of magnetization of the pinned layer is caused by an exchange coupling with an anti-ferromagnetic layer provided adjacent to the pinned layer, wherein the anti-ferromagnetic layer acts as a pinning layer to the pinned layer.
With ever-increasing trend of recording density in the technology of disk storage devices, the importance of giant magneto-resistive sensor has increased evermore.
In a high-density magnetic disk of the future, a recording density of 40 Gbit/inch2 is projected. In such a high-density magnetic recording device, the magnetic disk carries recording tracks with a pitch of 57-80 kTPI, which corresponds to a track separation of 0.45-0.32 μm. In order to pick up magnetic signals from such high-density tracks, it is necessary to narrow the width (read-core width) of the giant magneto-resistive sensor to be 0.25 μm or less. In order to reduce the width of the giant magneto-resistive magnetic sensor, it is inevitable to apply a photolithographic process.
However, such a use of photolithographic process in the fabrication process of a giant magneto-resistive sensor raises a serious problem of oxidation of the anti-ferromagnetic layer used therein during the photolithographic process, which is conducted in the atmosphere. Further, the process of removing a resist mask may cause damage in the anti-ferromagnetic layer by the chemicals used for the removal of the resist mask.
Thus, it has been practiced in the art of giant magneto-resistive sensor of highly miniaturized width to protect the anti-ferromagnetic film in the photolithographic process by a metal cap film not reacting with the anti-ferromagnetic film, such as Ta.
FIG. 1 shows the construction of a miniaturized spin-valve sensor 10 according to a related art.
Referring to FIG. 1, the magnetic sensor 10 includes a magneto-resistive layer 13 for detecting a magnetic signal Hsig, wherein the magneto-resistive layer 13 has a standard layered structure of a spin-valve magnetic sensor and includes a ferromagnetic free layer, a ferromagnetic pinned layer, a conductive intermediate layer interposed between the free layer and the pinned layer, and an anti-ferromagnetic pinning layer provided on the pinned layer. For picking up magnetic signals Hsig from the magnetic tracks having an extremely miniaturized width, the magneto-resistive layer 13 also has a reduced width W. Such a miniaturized magneto-resistive layer 13 is obtained by conducting a photolithographic process as noted before. Thereby, it should be noted that the top surface of the magneto-resistive layer 13 of the construction of FIG. 1 is covered with a metal cap film such as Ta.
Further, the spin-valve sensor 10 of FIG. 1 includes a pair of domain-control regions 12A and 12B of a hard magnetic material disposed at both lateral sides of the magneto-resistive layer 13, and electrodes 11A and 11B are provided on the foregoing domain-control regions 12A and 12B, respectively. The spin-valve sensor 10 of FIG. 1 is called “abutted type sensor.”
In the construction of FIG. 1, it should be noted that the domain-control regions 12A and 12B have a predetermined magnetization 15 not responding to the external magnetic signal Hsig to the magneto-resistive layer 13 due to the large coercive force pertinent to a hard magnetic material, and the foregoing magnetization 15 of the domain-control regions 12A and 12B eliminates domain formation in the magneto-resistive layer 13, and hence, Barkhausen noise associated with the migration of the magnetic domain wall.
As a result, the magnetization 17 of the free layer in the magneto-resistive layer 13 changes the direction in response the external magnetic signal Hsig, and the magneto-resistance between the pinned layer and the free layer is changed accordingly. This change of the magneto-resistance is detected by causing to flow a sensing current 14 from the electrode 11A to the electrode 11B through the magneto-resistive layer 13 as represented in FIG. 1.
In the construction of FIG. 1, it will be noted that there are formed regions 16A and 16B in the magneto-resistive layer 13, more precisely in the free layer of the magneto-resistive layer 13, in which the direction of magnetization does not change in response to the external magnetic field Hsig, along the boundary to the domain-control region 12A or 12B. It should be noted that the magnetization 15 of the domain-control region 12A or 12B causes a pinning of magnetization in the free layer in correspondence to the foregoing regions 16A and 16B. Thus, the foregoing regions 16A and 16B form a dead zone. In view of the fact that the sensing current 14 flows through such dead zones 16A and 16B, the signal-to-noise ratio of the sensing current 14, and hence the sensitivity of the spin-valve magnetic sensor 10 of FIG. 1, is inevitably deteriorated. This problem becomes particularly conspicuous when the magneto-resistive region 13 has a reduced width W.
In order to overcome the foregoing problem, there is a proposal of a spin-valve magnetic sensor 20 according to a related art as represented in FIG. 2, wherein those parts corresponding to the parts explained with reference to FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 2, the spin-valve magnetic sensor 20 has a construction similar to that of the spin-valve magnetic sensor 10 of FIG. 1 except that the electrodes 11A and 11B are formed so as to extend over the magneto-resistive layer 13 to form overhang regions 28A and 28B respectively.
More specifically, the overhang region 28A of the electrode 11A extends beyond the dead zone 16A on the top surface of the magneto-resistive layer 13, while the overhang region 28B of the electrode 11B extends beyond the dead zone 16B on the top surface of the magneto-resistive layer 13. As noted previously, the top surface of the magneto-resistive layer 13 is covered by the metal cap film such as a Ta film.
According to the construction of FIG. 2, the sensing current 14 is caused to flow while avoiding the dead zones 16A and 16B, and the sensitivity of the magnetic sensor 20 is improved over the magnetic sensor 10 of FIG. 1. The spin-valve sensor 20 of FIG. 2 is called an “overlay type sensor.”
In the overlay type sensor 20 of FIG. 2, it will be noted that the sensing current is injected into the magneto-resistive layer 13 across the interface between the electrode 11A and the magneto-resistive layer 13 or the interface between the electrode 11B and the magneto-resistive sensor 13. Thus, the electric property of the metal cap film, typically a Ta film, provided on the top surface of the magneto-resistive layer 13 becomes important in the overlay type magnetic sensor 20 of FIG. 2. As noted previously, such a metal cap film is provided to protect the anti-ferromagnetic pinning layer in the magneto-resistive layer 13 during the photolithographic process. As a photolithographic process includes various processes conducted in the atmosphere such as resist process, there is a substantial risk that the surface of the metal cap film is oxidized. In the case of a Ta cap film, for example, there is a possibility that the surface of the Ta cap film is covered by an oxide film of Ta2O5.
It will be understood that the existence of such an oxide film on the surface of the metal cap film increases the resistance of the sensing current path, and the signal-to-noise ratio of the magnetic sensor is deteriorated. In addition, there is a possibility that the sensing current 14 avoids the oxide film and flows along the path of FIG. 1. In this case, the sensing current 14 flows through the dead zones 16A and 16B and the signal-to-noise ratio of the magnetic sensor is deteriorated.
In addition, the abutting type sensor 20 of FIG. 2 has another problem, in relation to the fabrication process thereof, in that the metal cap film of Ta may be etched during the etching process to form the electrodes 11A and 11B. Thus, when there is an excessive etching during the etching process for patterning the electrodes 11A and 11B, the anti-ferromagnetic layer constituting a part of the magneto-resistive layer 13 may be damaged. In such a case, the exchange coupling magnetic field Hua caused by the anti-ferromagnetic layer would be influenced and the magneto-resistance of the magnetic sensor may be degraded seriously.